Restore guest item's original behavior in user switcher

Restore behavior of the guest item in the keyguard user switcher to it's
original behavior:

 - When signed in as guest, the guest item says "Remove guest"

 - When no guest account exists, the guest item says "Add guest"

 - When the guest exists but the current user is not guest, the guest
   item says "Guest"

Additionally:

 - Fix behavior of config_expandLockScreenUserSwitcher and
   lockscreenSimpleUserSwitcher

 - Remove "End guest session" button from keyguard user switcher

 - Remove "End guest session" string from SystemUI

Test: Open keyguard user switcher as guest user
Bug: 169783558
Bug: 181358587
Change-Id: Ibce9b3c7f28467e199db067b4bfb36b9fa35e4b4